Method of producing chromium steel castings



Number 553,839.

0 steels favorably and to Patented Feb. 12, 1935 ENTI OFFICE tmrnon orrnonuome CHROMIUM STEEL cns'rmcs RussellFi-anks, Jackson Heights, N. Y., asslgnor to Electra .Metallurgical Company, a corporation'ot West I 1N0 Drawing. Original Virginia v application July 29,

Serial No..553,889. Divided'and this application June 17, 1932, Serial No. 617,920

2 Claims. '(CI. 75-45) My inventionrelates to chromium steels, re-

ferring more specifically to a means of improv-.

ing such steels. This application is a division of my prior applicationfiled July 29, i931, Serial It is the objectoi my, invention to improve the physical properties of alloy steelscontaining a high percentage of chromium, and I attain this object by applying my discoverythatthe addition of nitrogen in alloying quantities appreciably greater than those normally present in steels aifects thephysical properties of high-chromium a marked degree.

It is a matter of common observation and experience that cast high-chromium steels: consist of coarse crystals and grains loosely bound together. actually porous, as shown by hydraulic: tests at high pressure. This coarse structure in cast'highchromium steel spcnsible for the comparatively poor physical properties .of the metal. grain-structure isgreatly to bedesired for the production of steels with superior physical properties.

I have found that the amounts greater than about 0.2% materially improves the physical properties of wrought and cast steels which contains. large percentage of ii chromium. Although wrought chromium steels do not have the coarse grain-structure. of the cast metal, the application of duces marked improvements over the normally good structure of these steels. My invention is therefore applicable not only to cast, but also to wrought chromium steels, and produces a superior 5 product in either case.

According to my invention, some benefit may be had by the use of a nitrogen content somewhat less than 0.2%, but it is preferable that the nitrogen content be at least this value. In general, it is desirable to have the maximum amount of nitrogen present that can be retained in stable combination at the temperatures which are required for casting the metal. The amount of nitrogen which can be retained in the final product increaseswith increased chromium content; thus, a steel'containing about 15% chromium will retain up to approximately 0.2% nitrogen, while a steel containing about 35% chromium will retain up to approximately 0.65% nitrogen. The addition of large amounts of nickel will lower somewhat these upper limits of nitrogen solubility.

My invention is especially valuable when applied to steels containing from about 15% to 35% steels generally.

Castings of this metal areweak and is "to. aconsiderable extent re- A refinement of thepresence of nitrogen in 0.1%. of; carbon, about my invention proderived from the 'use of nitrogen. This invention is therefore Experiments which I have madedemonstrate that the addition of nitrogen in amounts apprecivbly greater than those normally present in steels refines the grain-structure and improves the strengthof high chromium steels without materially' aifectihg their ductility, machinability,

hardness, corrosion resistance, or forging, rolling and working, characteristics. For instance, my tests show that the addition of about 0.2% to about 0.4% of alloyed nitrogen to wrought and cast chromium steels containing from about 18% to about 30%chromium and up to about 2% carbon greatly improves thephysical properties of these steels. nitrogen is most 'marked in those steels containing less than about 1% of carbon, but continues' to be appreciable when the carbon is raised as high as'about,2%.fl v

, In one series of experiments a number of wroughtsteelltestsamples were made, each sample compriSi-ngQapreponderance of iron, about 0.4% manganese, about 0.5% silicon, a low percentage of impurities, and added chromium and nitrogen. The nitrogen was introduced into themolten steels in the form of chromium nitride and a high nitrogen ferrochrome alloy. For eachhigh-nitrogensample so .made there was prepared a corresponding low nitrogen steel having composition in respect approximately the same toevery constituent except nitrogen. All of the samples were then subjected to a number of physical tests under comparable conditions. The effect of nitrogen as indicated by these tests is most conveniently by means of the following table:

applicable. to high chromium The beneficial effect of theadded shown In the abo e table of wrought steels, the following symbols are used: Y. P. for yield point in thousands of pounds per square inch, T. S. for tensile strength in thousands of pounds per square inch, El.% for percentage elongation obtainable in 2 inches, R. A. for percentage reduction in area of cross section accompanying the elongation, Eric, for the values obtained. using the Erichsen machine, and Brinell for the hardness valueson the Brinell scale.

A microscopic examination of the wrought steels showed that nitrogen greatly refines the structure of the metal and appears to eliminate the grain boundaries of the crystals. Further testing of the steels demonstrated that the addition of nitrogen decreases their tendency to become embrittled at elevated temperatures, such as those of the order of 415 C.

A corresponding series of tests were made on cast high chromium steels. A series of castings containing a preponderance of iron, a low percentage of impurities, about 15% to 35% chromium, about 0.5% of carbon, and with high and low nitrogen contents, were cast in the form of standard arbitration bars and tested. Nitrogen was found to improve the physical properties and refine the grain-structure greatly. A typical set of values obtained from this series 01' tests follows:

In the above table for cast steels, the following symbols are used: Dell. for deflection in inches, T. B. L. for transverse breaking load in thousands of pounds, and Brinell for the hardness values on the Brinell scale.

The presence of nitrogen in no way decreases the resistance of chromium steels to staining and certain types of corrosion, as I have determined by extensive tests of both wrought and cast steels.

In making all castings used in my tests, I have followed a new and special procedure which I have developed for applying my invention. A low carbon steel, according to my procedure, is melted in one of the usual types of furnaces. During the melting process the desired amount of nitrogen is introduced into the molten metal as chromium nitride or a high nitrogen ferrochrome alloy or other suitable nitride or mixture of nitrides. After the steel is melted it is held at a temperature sumciently above the melting point to allow the nitrogen liberated from the unstable nitrides to escape. By regulating the temperature of the molten steel, the content of the combined stable nitrides can be controlled to give solid cast metal, as the stability of the nitrides decreases with a rise in temperature. This method of producing high nitrogen steels avoids the formation of blowholes and excessive piping, since the nitrogen remaining in the steel is in an alloyed condition.

The above-mentioned chromium nitride may be prepared in any appropriate manner, for example, by heating finely-divided chromium metal in nitrogen or ammonia at a temperature approximately 850 Although I have described a number of specific examples and disclosed the preferred form of my invention, it will be understood that these are merely representative embodiments, and do not tend to limit the scope of my real discovery and invention relative to amethod generally applicable to the improvement of alloy steels containing a high content of chromium. I wish therefore to be limited only by the prior art and the appended claims.

I claim:

1. The method of producing high-chromium steel castings which comprises melting the metallic constituents, adding thereto nitrogen in the form of metallic nitride in an amount more than that which will combine with: the steel at the melting-point thereof, holding the fluid at a temperature oi substantial superheat for a time sumcient to allow the nitrogen content of the fluid to decrease at least to a percentage which can be stably held in combination by the steel when cast, and allowing said fluid to solidify.

2. The method of producing alloy steel castings containing from about 15% to about 35% of chromium, up to about 2% of carbon, a preponderance of iron and at least 0.2% .of nitrogen which comprises melting a carbon'steel, adding thereto nitrogen and chromium in the form of chromium nitrides and highenitrogen ferrochrome in such amounts and proportions that the chromium is present in the molten metal in the desired percentage and the nitrogen is present in an amount more than that which will combine with the steel at the melting-point thereof, holding the fluid at a temperature of substantial superheat for a time sufllcient to allow nitro en content of the fluid to decrease at least to a percentage which can be stably held in combination by the steel cast, and allowing said fluid to solidify.

RUSSELL FRANKS. 

